This invention relates to semiconductor chip packaging.
Various surface mount package configurations are known. Each package type supports an integrated circuit chip or die, and provides protection from hostile environments and enables interconnections between the die and a printed circuit board.
FIG. 1 shows a conventional non-leaded molded integrated circuit package, generally at 10. A lead frame 30 is the supporting structure of the molded integrated circuit package 10 to which the other elements are attached. The lead frame 30 is made from a thin metal strip by etching or stamping to form a pattern of leads 32 around a die attach paddle 34. A die 40 is mounted upon an xe2x80x9cupperxe2x80x9d die support surface 35 of the die paddle 34 using a die attach material 50, which is typically an epoxy resin. The die 40 has a xe2x80x9clowerxe2x80x9d surface 42 in contact with the die attach material 50 and an opposing xe2x80x9cupperxe2x80x9d surface 44. Electrical connections between the die circuitry and the leads 32 are provided by fine gold wires 60, which are bonded to die bond pads 46 on the die 40 at the upper die surface 44 and to wire bond surfaces 36 on the leads 32 of the lead frame using a wire bond technique. The lead frame 30, die 40 and wires 60 are covered with a thermoset plastic encapsulation or casing 70 using transfer molding.
FIG. 2 shows another conventional non-leaded molded integrated circuit package, generally at 20. In this configuration, as in the package shown in FIG. 1, a lead frame 30 includes a pattern of leads 32 around a die attach paddle 34. A die 40, having lower and upper surfaces 42, 44 is affixed to a die support surface 35 on the die paddle 34 using a die attach material 50. Fine gold wires 60 are bonded to die bond pads 46 at the upper die surface 44 and to wire bond surfaces on the leads 32 by wire bonding. In this configuration, electrical connections between the die circuitry and the die paddle 34 are additionally provided by fine gold wire down bonds 62, which are bonded to die bond pads 46 at the upper die surface 44 and to down bond surfaces 38 near the margin 37 of the die paddle 34. The lead frame 30, die 40 and wires 60 and 62 are covered with a thermoset plastic casing.
The integrated circuit packaging industry is always driving toward more reliable packages, and smaller, thinner and more lightweight packages. The lead frame thickness of conventional non-leaded packages limits reduction of package thickness, and conventional non-leaded packages are less reliable than are other package types. One approach to reducing package thickness is to eliminate a central die attach paddle, as is shown for example in Mostafazadeh et al. U.S. Pat. No. 5,894,108. An approach to improving structural reliability is to add lips on the die pad edge, as is shown for example in Glenn U.S. Pat. No. 6,143,981. Reduction of lead frame thickness results in an undesirable loss of frame stiffness. Reduction of the thickness of the plastic encapsulation can result in reduced package reliability. Formation of lips on the die pad edge can result in sharp edged lips that can act as initiation points for propagation of cracks in the encapsulation or of delamination of the encapsulation from the lead frame, which can disrupt the electrical connection (particularly at the down bond or wire bond attachment sites) and deteriorate package reliability.
To reduce overall package thickness, the thickness of the encapsulation may be reduced; or the die may be made thinner; or a thinner lead frame may be employed. Reduction of package thickness conventionally entails making compromises, either in the manufacturability (and therefore the manufacturing cost) or the reliability of the package. Generally, a thinner die has poorer resistance to die cracking. And thinner lead frames generally are more difficult to handle.
The invention is directed to lead frame configurations providing for lead frame surface mount packages having improved structural integrity and electrical reliability.
Generally according to the invention, the die supporting surface of the die attach paddle is on a plane at a different level from the down bonds attachment surface, or may be on a plane at the same level, but separated at least partly by a slot (which may be a groove). The die support surface may be recessed below the plane of the down bond attachment surface, or the die support surface may be in a plane with the down bonds attachment surface, but separated at least partially by a slot or recess in the plane. In configurations where the die attach surface is on a plane at a different level, a barrier between the die attach region and the down bonds attach site is provided by an edge of the down bonds attachment surface (if the plane of the die attach surface is lower) or by an edge of the die attach surface (if the plane of the die attach surface is higher). This barrier limits flow of the die attach epoxy outward from the die attach region to the down bonds attach site during the die attachment process. Or, in configurations where a recess or slot is formed on the paddle surface, situated between the die attach region and a down bond attachment site, the recess interrupts the surface at the down bonds attachment site from the surface at the die attach region, and serves as a moat for collection of die attach epoxy that may flow outward from the die attach region during the die attach process.
According to the invention, the integrity and reliability of down bonds is improved by inhibiting epoxy bleed out to the down bonds attachment sites.
Also generally according to the invention, portions of the lead frame, particularly on the paddle near the paddle margin, or on the leads, are sculptured in various configurations to provide improved interlock between the encapsulation and the lead frame. The configurations which may for example include grooves, slots, scallops, and recesses, are designed so as to avoid formation of sharp edges (that is, to avoid formation of acute edges), which may serve as initiation sites for propagation of cracks in the encapsulation or delamination of the encapsulation from the lead frame. These various features according to the invention provide various mechanical functions such as mold locking, prevention of die attach epoxy resin bleed and delamination propagation. Encapsulation material can be injected into the sculptured features during package assembly, and serve to hold the lead frame and package body together and to provide resistance from package cracking and delaminating. Certain of the various sculptured features can additionally serve as a dam or moat to prevent overflow of die attach into the down bonds attachment sites during the die attach process.
Configurations having a recessed die attach region provide for a thinner overall package without substantially compromising the mechanical properties or handling qualities of the lead frame. Moreover, by thinning the die paddle without entirely eliminating it at the die attach region, the die paddle may be soldered directly to an underlying circuit board, to provide for efficient heat dissipation from the die.
Because the invention provides for improved reliability of electrical connections to the lead frame, the invention also provides for reliable manufacture of thinner and more reliable packages at lower cost. The configuration of the slots or depressions in the paddle and lead surfaces provide for a thinner overall package profile without compromise of structural integrity or of reliability of electrical connections.
Any of the various lead frames of the invention can be made by a conventional manufacturing process, usually a chemical process such as etching, using conventional materials and process machinery. Other processes, for example a mechanical process (such as drilling or stamping), that may be employed may be less efficacious than etching.
The invention features a lead frame for a surface mount semiconductor chip package, including a die attach paddle and leads. The die attach paddle has a peripheral margin having down bond attachment sites on an upper surface of the paddle near the margin, and having a central die attach region on an upper surface of the paddle. In one general aspect of the invention, a portion of the upper surface of the paddle is recessed to provide a site for accumulation of any die attach material that may during a die attach process flow from the die attach region toward any of the down bond attachment sites.
In some embodiments the recessed portion of the upper surface of the paddle includes the die attach region. In some embodiments the recessed portion of the upper surface of the paddle includes a groove or slot situated at least in part between at least one of the down bond attachment sites and the die attach region; in some such embodiments the recessed portion includes a groove arranged substantially parallel to the peripheral margin of the paddle; in other such embodiments the recessed portion includes a groove at least partly surrounding at least one of the down bond attachment sites. In some embodiments the recessed portion of the upper surface of the paddle includes a plurality of recesses extending from near the peripheral margin of the die attach paddle inward toward the die attach region; in some such embodiments the recesses extending from near the margin alternate with nonrecessed portions of the surface, each having one or more down bond attachment sites; in some such embodiments the non recessed portions of the surface extending outwardly beyond a base margin of the paddle to form cantilever portions, and in some such embodiments the down bond attachment sites are on the cantilever portions. In some embodiments in which the recessed portion includes a groove, a depth of the groove is at least approximately half a thickness of the paddle; in some such embodiments, the groove is a slot passing through the entire thickness of the paddle.
In another general aspect, the invention features a nonleaded molded package having a second die stacked over a first die attached on a die attach region of a surface of a lead frame. In some embodiments a spacer, such as a silicon spacer, is interposed between the first die and the second die in the stack.